This invention relates to wells drilled in the earth and more particularly to identifying subsurface formations penetrated by such wells which are hydrocarbon bearing.
In the drilling of wells into the earth by rotary drilling techniques, a drill bit is attached to a drill string, lowered into the well, and rotated in contact with the earth, thereby breaking and fracturing the earth and forming a wellbore. A drilling mud is circulated down the drill string and through ports provided in the drill bit to the bottom of the well and then upward through the annular space formed between the drill string and the wall of the well. The drilling mud serves several purposes including cooling the drill bit, removing formation cuttings from the well, and supplying hydrostatic pressure upon the formations penetrated by the well to prevent fluids existing therein under pressure from flowing into the well.
Customarily during the drilling of the well a mud log is made. This log is made by analyzing the drill mud for the presence of hydrocarbon gas in the entrained formation cuttings removed from the wellbore from subterranean hydrocarbon bearing formations. The location of the drill bit is correlated with the mud analysis to provide a depth reference to the mud log.
Such a mud log provides important depth information for use in completing the well for the production of oil and gas, especially in the perforation of the well after casing. More particularly, a string of casing is often run into the well and cement is poured into the annulus between the casing string and the wall of the well to form a cement sheath for bonding the casing to the well. A perforating means is then lowered into the well adjacent the subterranean formation to be produced and activated to perforate the casing and cement sheath to provide fluid communication tunnels between the interior of the well and the subterranean formation to be produced. The mud log aids in the positioning of the perforating means in the well by identifying the depth in the well at which formation cuttings having entrapped hydrocarbon gas were obtained.